1. Field of the Invention
The present invention relates to a holder of optical transmission lines and a holder for a multi-core optical wave-guide.
2. Description of the Related Art
Recently, optical communication techniques and optical transmission technologies in which signals are transmitted by a carrier wave, modulated by intensity modulation or phase modulation, etc., has been widely adopted. For facilitating such optical transmissions, an optical semiconductor module for optically coupling optical transmission lines such as a bundle of optical fibers to an optical device chip, which integrates a plurality of optical semiconductor elements such as light-emitting elements and/or photo-detecting elements, with a high-precision position controlling technology are required.
As optical signals transmitted in the optical semiconductor module become high-speed, dimensions of the light-emitting region and the light-receiving region of the elements must decrease because parasitic capacitances of light-emitting elements and photo-detecting elements cannot be ignored. For instance, a diameter of light-receiving face of GaAs based pin photodiode is miniaturized to about 50–60 μm so as to obtain a response in a region over 10 Gbps. With such a small light-receiving region, optical coupling efficiency decreases, because a light beam emitted from a multimode optical fiber expands larger than the diameter of the light-receiving face of the photo-detecting element. The decrease in the optical coupling efficiency deteriorates noise-proof performance, and problems such that the signal cannot propagate enough distance is caused.
Further, so as to take enough tolerance of the displacement of the relative position of the optical semiconductor elements, which are integrated with an array configuration in the optical device chip, against corresponding cores of the optical fibers, a lens is inserted in the optical paths. However, the insertion of the lens increases the number of packaging components, which makes controlling of the position more and more difficult, and the packaging cost tends to rise.
Then, for the purpose of reduction of the packaging cost, direct optical coupling architecture called “butt joint”, in which lights are directly coupled into an optical fiber, by disposing the optical fiber close to the optical device chip so the optical fiber faces to the optical device chip, without using a lens, has been researched and developed. In the direct optical coupling architecture, because light emitted from the optical semiconductor element or light emitted from the optical fiber transmit through an intervening medium having a substantially homogenous refractive index such as air and/or refractive index matching material, which has no wave-guiding characteristics, the beam of the emitted light expand in the intervening medium. Therefore, the relative portion of light which reaches another area other than the targeted wave guiding region (core) of the optical fiber or the targeted active region of the photo-detecting element increases so as to decrease the optical coupling efficiency, which deteriorates the noise-proof performance. In addition, different kinds of noises such as crosstalk noise increase with the increase of stray light, and an adverse effect may be caused in the signal transmission. Therefore, a configuration where the optical fiber is disposed closer and closer to the optical device chip becomes important so that light emitted from the optical fiber does not reach another area other than the targeted region.
For instance, light emitted from a multimode optical fiber having a numerical aperture (NA)=0.21 and a diameter of 50 μm establishes a divergent angle of about 12 degrees in the air. Therefore, the distance from the optical fiber to the optical device chip must be as close as several decade μm.
Therefore, a holder, which is also called “an optical fiber ferrule”, configured to hold optical fibers into sleeves formed in the holder is proposed, so that a plurality of electrical interconnections are delineated directly on the main face of the holder. The holder mounts an optical device chip on the main face, and a plurality of optical fibers are accommodated by the holder so that the end faces of the optical fibers can face to the corresponding active regions (emitting/receiving regions) of the optical device chip. With the optical fiber ferrule, the emitting/receiving elements can be assembled very close to the end face of the optical fiber. And since optical semiconductor elements can be assembled directly on the optical fiber ferrule using the location of the optical fibers as a reference location, a package having a high accuracy in the lateral direction, suppressing an increase in the number of components, and suitable for low-cost packaging, using an usual flip chip packaging, can be provided. In addition, the manufacturing cost of the optical fiber ferrule can be drastically reduced by using resin for the substrate material of the holder. And by delineating the electrical interconnections from the main face where a plurality of openings of the sleeves for the optical fibers are cut over to a side face, orthogonal transformation of the plane is achieved so that the direction along which the optical fibers extend and the mounting face of the optical fiber ferrule are in parallel, thereby preventing a configuration in which the optical fibers extend perpendicular to the mounting face. However, according to the configuration of the earlier technology, a heat conduction passage for heat generated in the emitting/receiving elements are only electrical interconnections designed for signal extraction, although thermal flow can be achieved by heat radiation into the air. Especially, when the substrate material of the optical fiber ferrule is made of resin, thermal transport can hardly be ensured, since heat radiation to a substrate material of the holder is extremely bad. Therefore, measures such as installing a heat conduction passage from outside to a back surface of the optical device chip in which the optical semiconductor elements are merged, for example, is required, which increases the manufacturing cost. And such a problem becomes serious in the case of “an optical semiconductor device array” in which a plurality of optical semiconductor elements are integrated in an optical device chip. Since a semiconductor substrate (semiconductor chip) which merges optical semiconductor elements, is made from a comparatively low thermal resistance material, each of the optical semiconductor elements integrated in a single optical device chip thermally interfere, and are susceptible to variations in mark densities (duty factors) and/or operating currents of adjacent optical semiconductor elements. It is very difficult to add external heat conduction passages in every optical semiconductor element so as to prevent thermal interference between the optical semiconductor elements.
In this manner, in a configuration in which electrical interconnections are delineated directly on a main face of the holder, and active regions (emitting/receiving regions) of the optical device chip are arranged to face optical fibers on the main face so as to couple optical fibers directly, as the heat conduction passages for heat generated in the optical semiconductor elements, only electrical interconnections for signal extraction and heat radiation into the air can be utilized. However, since extending the length of electrical interconnections increases capacitance, inductance, and/or resistance associated with the interconnections so as to deteriorate the performance of the optical fiber ferrule, it is impossible to extend the length of interconnections over the required minimum length. Therefore, a sufficient heat radiation effect cannot be expected. Especially, in the case where the holder (optical fiber ferrule) is made of resin, means for thermal transport is hardly ensured, since heat radiation to a substrate material of the holder is extremely bad. The methodology of installing external heat conduction passages to the back surface of the optical device chip in which the optical semiconductor elements are merged, etc might be acceptable for ensuring means for thermal transport, but the installation of the heat conduction passages to the back surface increases the manufacturing cost. Such a problem becomes serious for the optical semiconductor device array in which a plurality of optical semiconductor elements are integrated in a single optical device chip. Since thermal resistance of a semiconductor substrate (semiconductor chip), in which the optical semiconductor elements are merged, is comparatively low, each of the optical semiconductor elements monolithically integrated in the optical device chip thermally interfere mutually, and are susceptible to the variation of mark density and/or operating current of adjacent optical semiconductor elements.